1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a des red light image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs) and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
FIG. 1 shows a conventional LCD device. The LCD device 11 includes an upper substrate 5 and a lower substrate 22 with a liquid crystal layer 14 interposed therebetween. The upper substrate 5 and the lower substrate 22 are commonly referred to as a color filter substrate and an array substrate, respectively. Within the upper substrate 5 and upon the surface opposing the lower substrate 22, a black matrix 6 and a color filter layer 8 are formed in the shape of an array matrix and include a plurality of red (R), green (G), and blue (B) color filters so that each color filter is surrounded by corresponding portions of the black matrix 6. Additionally, a common electrode 18 is formed on the upper substrate 5 to cover the color filter layer 8 and the black matrix 6. In the lower substrate 22 and upon the surface opposing the upper substrate 5, a thin film transistors (TFTs) “T” are formed in an array matrix corresponding to the color filter layer 7. A plurality of crossings gate lines 13 and data lines 15 are positioned such that each TFT “T” is located adjacent to each crossover point of the gate lines 13 and the data lines 15. Furthermore, a plurality of pixel electrodes 17 are formed on a pixel region “P” defined by the gate lines 13 and the data lines 15 of the lower substrate 22. The pixel electrode 17 includes a transparent conductive material having good transmissivity such as indium-tin-oxide (ITO) or indium-zinc-oxide (ITO), for example.
In the LCD device 11 of FIG. 1, a scanning signal is applied to a gate electrode of the TFT “T” through the gate line 13, while a data signal is applied to a source electrode of the TFT “T” through the data line 15. As a result, the liquid crystal molecules of the liquid crystal layer 14 are aligned and arranged by operation of the TFT “T,” and incident light passing through the liquid crystal layer 14 is controlled to display an image.
As described above, since the pixel and common electrodes 17 and 18 of the conventional LCD device are positioned on the lower and upper substrates 22 and 5, respectively, the electric field induced between them is perpendicular to the lower and upper substrates 22 and 5. The described liquid crystal display device has advantages of high transmittance and a high aperture ratio. Furthermore, because the common electrode 18 on the upper substrate 5 acts as a ground, the liquid crystal is shielded from static electricity.
However, the conventional LCD device having the longitudinal electric field has a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring, to FIGS. 2 and 3A to 3D.
As shown in FIG. 2, lower and upper substrates 30 and 32 are spaced apart from each other, and a liquid crystal layer 10 is interposed therebetween. The lower and upper substrates 30 and 32 are often referred to as an array substrate and a color filter substrate, respectively. On the lower substrate 30 are a pixel electrode 34 and a common electrode 36. The pixel and common electrodes 34 and 36 are aligned parallel to ea(h other. On a surface of the upper substrate 32 is a color filter layer 42 that is commonly positioned between the pixel electrode 34 and the common electrode 36 of the lower substrate 30. An overcoat layer 44, which protects the color filter layer 42, is formed on the color filter layer 42. A voltage applied across the pixel and common electrodes 34 and 36 produces an electric field 35 through the liquid crystal “LC.” The liquid crystal “LC” has a positive dielectric anisotropy, and thus it aligns parallel to the electric field 35. An edge sealant 40 is formed around the edges of the lower and upper substrates 30 and 32, and bonds the upper substrate 32 to the lower substrate 30 to prevent leakage of the liquid crystal “LC”.
FIGS. 3A to 3D conceptually help illustrate the operation of a conventional IPS-LCD device. When no electric field is produced by the pixel and common electrodes 34 and 36, i.e., off state, as shown in FIGS. 3A and 3B, the longitudinal axes of the LC molecules “LC” are parallel and form a definite angle with the pixel and common electrodes 34 and 36. For example, FIG. 3B shows a common angle of 45 degrees between a line that is perpendicular to the pixel and common electrodes 34 and 36 and the longitudinal axes of the LC molecules.
On the contrary, when an electric field is produced by the pixel and common electrodes 34 and 36, i.e., on state, as shown in FIGS. 3C and 3D, beck use the pixel and common electrodes 34 and 36 are on the lower substrate 30, an in-plane electric field 35 that is parallel to the surface of the lower substrate 30 is produced. Accordingly, the LC molecules “LC” twist to bring their longitudinal axes into coincidence with the electric field. Thus, as shown in FIG. 3D, the LC molecules align with their longitudinal axes parallel with a line perpendicular to the pixel and common electrodes 34 and 36.
In the above-mentioned IPS-LCD panel, there is no common electrode on the color filter substrate. Furthermore, since the above-mentioned IPS-LCD panel has the pixel electrode and the common electrode on the array substrate, it uses the parallel electric field to the array substrate.
Now, referring back to FIG. 2, the overcoat layer 44 is formed )n the color filter layer 42 to cover and protect the color filter layer 42. Further, the edge seal, ant 40 is formed around the periphery of the IPS-LCD panel. Although the black matrix is not shown in FIG. 2, it is formed on the upper substrate 32 surrounding the color filter layer 42. Because the IPS-LCD device produces the in-plane electric field 35, the black matrix should be made of an organic substance and not a metallic material in order to prevent the distortion of the electric field.
Furthermore, there are some problems in the edge sealant 40 and the overcoat layer 44. In general, a number of ions are contained in the edge sealant 40. As time passes, these ions migrate into the liquid crystal layer 10 after the LCD panel is complete. In other words, since the edge sealant 40 is formed of a epoxy-based resin that has a great water resistance, the edge sealant 40 includes sodium ions (Na+), chlorine ions (Cl−), potassium ions (K+) and/or fluorine ions (F−), and these ions flow out as time passes. As these ions migrate through the liquid crystal layer 10, they deteriorate the liquid crystal layer 10 and a t to cause serious defects therein, thereby shortening life of the liquid crystal layer 10.
Moreover, the color filter layer 42 contains a number of ions, but the overcoat layer 44 prevents these ions from coming out from the color filter layer 42. However, the overcoat layer 44 also contains a number of ions. The ions in the overcoat layer 44 also migrate into the liquid crystal layer 10 as time passes, thereby accelerating the deterioration of the liquid crystal layer 10. Since the overcoat layer 44 is commonly made of an acryl-based resin, this overcoat layer 44 contains sodium ions (Na+), potassium ions (K+), iron ions (Fe2+/Fe3+), aluminum ions (Al3+), etc.
When the liquid crystal layer contains ions as described above, the driving voltage used to create the electric fields in the liquid crystal during operation of the liquid crystal is changed because of the presence of these ions. Accordingly, quality of the liquid crystal panel display degrades as the liquid crystal panel is used over time.
To overcome the display degradation caused by the migration of ions into the liquid crystal, an auxiliary line is used in a non-display area of the array substrate. FIG. 4 is a partial plan view of an array substrate illustrating a conventional in-plane switching mode liquid crystal display device that has such an auxiliary line. As shown in FIG. 4, the array substrate is divided into a pixel area “A” and a non-pixel area “B”.
In the pixel area “A”, a plurality of thin film transistors (TFTs) “T”, a plurality of pixel electrodes 36 and a plurality of common electrodes 34 are arranged. Additionally, pixels “P” including the pixel electrodes 36 and common electrodes 34 are arranged in the pixel area “A.” On the other hand, an electrostatic discharge device 33 and an auxiliary line 38 are arranged in the non-pixel area “B”. Furthermore, a plurality of data lines 41 are perpendicularly arranged in both the pixel area “A” and the non-pixel area “B”. Each data line 41 is connected to each data pad 43 in the non-pixel area “B”.
Within the configuration shown in FIG. 4, the auxiliary line 38 receives a signal that is applied to the common electrodes 34, such that the ions flowing from the edge sealant 40 and overcoat layer 44 are trapped by the auxiliary line 38. The electrostatic discharge device 33 between each pixel “P” and each data pad 43 is disposed at a distance of about 160 micrometers from the pixels “P.” The electrostatic discharge device 33 also discharges the static electricity occurring during the manufacturing processes. Although not shown in FIG. 4, repair lines and other lines for electric circuits are also arranged in the non-pixel area “B”.
Further in the array substrate shown in FIG. 4, although the patterned metal of the electrostatic discharge device 32 may not receive the electric signals, the patterned metal may have an induced potential when the patterned metal is exposed to the electric field. Therefore, the liquid crystals are deteriorated and the induced potential causes and accelerates the dielectric polarization of the liquid crystals. Further, the induced potential accelerates the polarization of impurities included in the liquid crystal layer. Accordingly, the liquid crystal molecules are misaligned in the array substrate and the image quality of the LCD device degrades. Namely, the polarization caused by the induced potential directly affects the arrangement of the liquid crystal molecules in the pixel area “A” and thus causes the degradation of the display quality.